Electronic circuits of various applications may include multiple inductors other than transformers sharing a single core. Using a common core may be employed for space and cost saving purposes rather than for the explicit purpose of coupling energy from one shared-core inductor to another. In fact, field coupling between shared-core inductors may yield undesirable consequences. For example, such coupling may complicate the sensing of current flowing through each of two shared-core inductors in a direct current (“DC”)-to-DC converter such as a multiphase buck converter.
FIG. 1A is a prior-art schematic diagram of a two-phase DC-DC converter output section including time constant matched current sensing circuits 105 and 108 associated with each of the uncoupled output inductors L1 110 and L2 120, respectively. The technique of sensing/measuring instantaneous current flow through an uncoupled inductor is described by Tateishi in U.S. Pat. No. 6,469,481, the latter incorporated herein by reference in its entirety. The terms “sensing” and “measuring” are used synonymously herein and mean obtaining a voltage analogue waveform that is instantaneously proportional to inductor current, whether or not magnitudes of particular points of the voltage waveform are reduced to numerical values.
Each current sensing circuit includes a series coupled sense resistor (e.g., the sense resistors R1 125 and R2 130 corresponding to the sensing circuits 105 and 108, respectively) and sense capacitor (e.g., the sense capacitors C1 135 and C2 140 corresponding to the sensing circuits 105 and 108, respectively). Each such series coupled RC sense network is coupled in parallel with a series combination of the corresponding inductor and a resistor representing the inductor's parasitic DC resistance (“DCR”) (e.g., DCR1 145 and DCR2 150 corresponding to the inductors L1 110 and L2 120, respectively). The time constant of each inductor is equal to the inductance L of the inductor divided by the DCR of the inductor. The time constant of each RC sense network is the resistance R of the sense resistor multiplied by the capacitance C of the sense capacitor. The inductor time constant L/DCR is matched to the sense network time constant RC if L/DCR=R*C.
FIG. 1B is a prior-art waveform diagram illustrating an example inductor current 165B. FIG. 1B also illustrates a corresponding time constant matched voltage analogue 160B of the inductor current 165B. The sense voltage 160A and 160B are accurate representations of the current 165A and 165B flowing through the inductor L1 110 if time constant matching is adhered to.
Accordingly, the sense resistor and capacitor (e.g., the sense resistor R1 125 and the sense capacitor C1 135) are chosen such that their time constant RC is equal to the corresponding inductor time constant (e.g., the inductance of L1 110 divided by the resistance of DCR1 145). So chosen, the voltage V_C1 160A and 160B seen across the sense capacitor C1 135 is instantaneously proportional to the current I1 165A and 165B flowing through the inductor L1 110. Likewise, the voltage V_C2 170 seen across the sense capacitor C2 140 is instantaneously proportional to the current I2 170 flowing through the inductor L2 120.
However, the simple time constant matched current sensing circuits 105 and 108 become considerably more complex if the inductors L1 110 and L2 120 are magnetically flux-coupled. Although such coupled inductor current sensing circuits are known, they suffer variously from high complexity and consequent large surface area requirements, difficulty in tuning for wide ranges of inductances and coupling coefficients, inaccuracy in the use for phase balancing due to offset and gain errors associated with cascaded amplifiers, and/or high quiescent current consumption when used in high switching speed applications.